This colour was early known (Genesis 38:28). It was one of the colours of the ephod (Exodus 28:6), the girdle (8), and the breastplate (15) of the high priest. It is also mentioned in various other connections (Joshua 2:18; 2Sam 1:24; Lamentations 4:5; Nahum 2:3). A scarlet robe was in mockery placed on our Lord (Matthew 27:28; Luke 23:11). "Sins as scarlet" (Isaiah 1:18), i.e., as scarlet robes "glaring and habitual." Scarlet and crimson were the firmest of dyes, and thus not easily washed out.
These dictionary topics are from M.G. Easton M.A., D.D., Illustrated Bible Dictionary, Third Edition, published by Thomas Nelson, 1897. Public Domain, copy freely.
Easton, Matthew George. "Entry for 'Scarlet'". "Easton's Bible Dictionary".